Spectral Splitting of High Harmonic Generation in Extreme High-power Chirped Pulses

High harmonic generation (HHG) from the interaction of intense laser field with gas has been vastly studied over the past three decades. The microscopic physics behind this process is well explained by the semi-classical three-step model [1]: An electron is first ionized, then accelerates along a classical trajectory, and finally emits an extreme-ultraviolet (XUV) photon when re-collides with its parent atom. This process repeats itself every half optical cycle, which results in a comb-like spectrum of odd multiples of the laser’s central frequency.

Introducing a temporal chirp to the laser pulse shifts the harmonic spectrum in proportion to the amount of chirp and laser intensity [2–4]. Since chirped pulses are also stretched in time, the intensity of the driving laser pulse of a given energy is lowered. This sets an energy-dependent limit on the maximal possible chirp for which the intensity of the pulse is above the HHG threshold. Here, we present the advent of spectral splitting of individual harmonics when an extreme amount of temporal chirp is applied to the laser pulse.

We demonstrated the dependence of the HHG spectrum on the laser pulse’s temporal chirp using the NePTUN 20 TW laser system at Tel Aviv University [5]. This system delivers 25 fs long pulses with a central wavelength of 800 nm, and up to 600 mJ in energy. The pulses were focused using a 600 cm focal length lens into a variable-length cell, which in these experiments was set to be 6 cm long and was filled with Ar.

 

We reproduce the behavior of measured spectra numerically, and attribute the observed spectral splitting to temporal modulations of the harmonic generation efficiency with the evolving phase mismatch along the focal axis inside the gas cell.  

1. P. B. Corkum, Phys. Rev. Lett. 71 (1993).

2. D. G. Lee et al., Phys. Rev. Lett. 87 (2001).

3. J. Zhou, et al., Phys. Rev. Lett. 76 (1996).

4. S. J. Goh et al., Opt. express 24 (2016).

5. E. Porat et al. Proc. of SPIE 11036 110360I (2019)